The present invention relates generally to a method for treating sludge, and more specifically relates to a sludge drying method that is able to treat sludge with different water contents and that enables the reuse of treated sludge.
The existing methods for drying sludge commonly used in China and overseas generally disposes centrifugally dehydrated sludge with a water content of 80-85% or sludge dehydrated by frame and filter press with a water content of 70-75 for direct drying. The specific steps of method 1 comprise the following: Scatter the dehydrated sludge or press the dehydrated sludge into strips first and then transport the dehydrated sludge to a drying apparatus for drying. After drying in the drying apparatus, the dried sludge is discharged as the final product of drying. Water content of the resulting dried sludge is generally around 10%. The specific steps of method 2 comprise the following: The dehydrated sludge is first transported to a sludge drying apparatus for scattering or pressing into strips; then, dry the dehydrated sludge in the drying apparatus. After drying in the drying apparatus, the dried sludge is discharged as the final product of drying. Water content of the resulting dried sludge is generally around 10%. The specific steps of method 3 comprise the following: Mix (remix) the dried sludge with dehydrated sludge to be dried for granulation first and then transport the mixture to a drying apparatus for drying. After drying in the drying apparatus, the dried sludge is discharged as the final product of drying. Water content of the resulting dried sludge is generally around 10%. Efficiency of evaporating water during the drying process varies with different drying apparatus and hence the time required for drying generally varies between 2 to 8 hours. The drying temperature of the drying apparatus for low temperature drying is generally 80 to 130° C. and the drying temperature for medium temperature drying is generally 200 to 400° C. High temperature drying is seldom used. The said methods above have some serious problems and the major one concerns the high operation costs which are hardly economically affordable. Besides, equipment investment is also high; locally built sludge treatment plants are difficult to perform orderly because the equipment and techniques overseas cannot be adapted to specific local conditions.
The main reason for the above problems is that sludge drying consumes excessive energy. Such over-consumption of energy is due to the excessive high water content of the dehydrated sludge to be dried, resulting in large amount of water to be evaporated during drying and dehydration. Due to the excessively high water content of the dehydrated sludge to be dried, the sludge to be dried increases in size and mass and therefore the amount of dehydrated sludge also increases. As a result, the workload of sludge drying equipment increases while the amount of sludge to be treated by the sludge drying equipment per unit time also increases. Since sludge with a water content of 60-80% is more viscous and difficult to be scattered, heat transfer and mass transfer are hindered. Sludge with a water content of 40-60% is the most viscous and the most difficult to be scattered, so mass transfer and heat transfer of sludge drying are hindered the most. Furthermore, sludge added with PAM before dehydration and then dehydrated by way of centrifugal dehydration causes uniting and twisting sludge floc. The twisted sludge floc locks water inside the sludge and makes it difficult to separate and remove water from the sludge.
Patent application number 200610113411.7 with the publication number CN1931753A published on 21 Mar. 2007 and entitled “A method for drying and incinerating sludge” discloses a method which comprises the following steps: Step 1, heat sludge with a water content of 95% to 40-60° C. and then dehydrate the sludge in a mechanically operated dehydration apparatus to lower the water content of the sludge to 65-80%. Step 2, granulate the sludge to form round particles each of a diameter of 1-3 mm. Step 3, mix the granulated sludge with dry sludge powder for the dry sludge powder to adhere onto the surface of the granulated sludge. Step 4, heat and dry the granulated sludge in a fluidized bed and separate it from heating gas. Step 5, increase the temperature to 50-60° C. before incineration and then incinerate the granulated sludge in an incinerator. Part of the heat produced by incineration is used by a drying apparatus for drying wet sludge. As sludge with a water content of 95% is first heated to 45-60° C. and then mechanically dehydrated, energy consumption is significantly increased. It is because sludge with a water content of 95% is very great in size and mass due to its extremely high water content. Such sludge at least doubles in size and mass in comparison with dehydrated sludge with a water content of 80%. A significant amount of energy is therefore wasted for heating up such sludge. Besides, the high impurity content of local sludge often causes damages to the heat pipe when the sludge is dried by indirect heating using a heat pipe fluidized bed, therefore hindering normal operation. Also, as sludge with a water content of 75-80% is very viscous and difficult to granulate, granulated particles stick to one another very easily.
Patent application number 20061006106.2 with the publication number CN1884154A published on 27 Dec. 2006 and entitled “A method of drying a sludge carrier” discloses a method which comprises the following steps: 1, Pre-treat the sludge by way of deodorizing the sludge and passivating heavy metals therein. 2. Mix the pre-treated sludge with dry sludge powder in a mass proportion of 1 to 1-5 and then high speed stir for conditioning at a speed of 50-150 revolutions per minute and a linear velocity of 78.54-235.62 m per minute. 3. Dispose the conditioned sludge in a curing area for aerobic curing by turning it over once in 6-10 hours until the water content of the conditioned sludge is lowered to 20-25%. 4. When the conditioned sludge is cured, it is disposed for drying in a hot air system for 6-10 minutes under a hot air input of 10,000-15,000 m3 per hour and the output dry sludge powder is controlled at a water content of less than 10%. This method requires large area for aerobic curing and it is significantly affected by weather conditions for sludge to be naturally dried to attain a water content of 20-25%.